Polymer International Polym Int 55:1013–1020 (2006)

Structural characterization and masstransfer properties of polyurethane blockcopolymer: influence of mixed soft segmentblock and crystal melting temperatureSubrata Mondal∗ and JinLian HuInstitute of Textiles and Clothing, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong

Abstract: An attempt has been made to investigate the influence of mixed soft segment on structure andmass transfer properties of segmented polyurethane (SPU). For this purpose polyurethane block copolymercontaining soft segment such as polycaprolactone glycol (number-average molecular weight 3000, PCL 3000), PCL3000–polypropylene glycol (number-average molecular weight 3000, PPG 3000), PCL 3000–polytetramethyleneglycol (number-average molecular weight 2900, PTMG 2900), PPG 3000–PTMG 2900, were synthesized using atwo-step or three-step synthesis process. All the SPUs were modified with the hydrophilic segment, i.e. diol-terminated poly(ethylene oxide) (number-average molecular weight 3400, PEG 3400). Fourier-transform infrared,wide-angle X-ray diffraction, differential scanning calorimetry, and dynamic mechanical thermal analysis wereused to characterize the polyurethanes. The mass transfer properties were measured by equilibrium sorption andwater vapor permeability measurements. Mixed blocks loosen the inter-chain interaction due to phase mixingwhich decreases the crystallization of the soft segment in the resulting SPU. The crystallinity of mixed polyol blockSPU increases when both polyols are crystallizable in the pure state. Highest loss tan δ value was observed forthe sample containing PTMG 2900–PPG 3000 mixed soft segment due to their flexible and phase mixed structurewhich increases the chain mobility; this sample performed best among all four SPUs in equilibrium water sorptionas well as water vapor permeability owing to their loose and nearly amorphous structure. Soft segment crystalmelting further enhances the water vapor permeability significantly, which would make the membrane suitablefor breathable textiles, packaging and medical applications. 2006 Society of Chemical Industry

Keywords: polyurethane block copolymer; mixed soft segment block; equilibrium water sorption; water vaporpermeability

INTRODUCTIONMultiphase segmented polyurethane (SPU) is com-posed of flexible soft segment and rigid hard segment.The variation in the composition of the soft and hardsegments imparts SPU films a wide range of physicalproperties. These copolymers have been used exten-sively in making elastomers, coatings, and foams.Physical, chemical, and permeability properties ofSPUs are directly related to the chemical composi-tion of their backbones, which are strongly dependenton the composition, type and molecular weights ofsoft segments. Thermal and mechanical propertiesand permeability of polyurethanes can be designedin a tailor-made fashion by changing their relativecompositions and lengths of constituent blocks.1–5

In breathable laminated/coated fabrics, the watervapor permeability of non-porous SPU films hasbecome an important element. Polymer-penetrantinteraction and primary structure of the polymeritself are important for understanding of non-porousmembrane functions such as sorption, diffusion,

and permeability of water vapor molecules.6 Stronginfluences of soft segment chemical nature on watersorption and permeability have been reported inthe literature.7,8 Schneider et al.7 studied the watersorption and permeability for four PUs of X /MDI(4,4′-diphenylmethane diisocyanate)/1,4-BDO (1,4-butane diol) with different types of soft segment(X = PEO 2000 or PTMG 2000 or PPO 2000 orPBA 2000). PU containing soft segments of PTMG2000, PPO 2000 and PBA 2000 has extremelylow water vapor sorption at 21 ◦C (typically G <

3 wt%). As expected from these very low watersorption values, these PUs were poorly permeableto water vapor (normalized water vapor fluxes lessthan 1.5 kg µm m−2 h at 21 ◦C). On the other hand,PEO 2000-based PU shows much higher watersorption (G = 113%) and permeability (normalizewater fluxes, J = 165 kg µm m−2 h) at the sameoperating temperature. Hsieh et al.8 observed verylow water sorption and permeability at ambienttemperature for PUs of Y /MDI/1,4-BDO with soft

∗ Correspondence to: Subrata Mondal, Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong KongE-mail: subratamondal@yahoo.comContract/grant sponsor: International Postgraduate Scholarship of the Hong Kong Polytechnic University(Received 18 June 2005; revised version received 11 September 2005; accepted 1 November 2005)Published online 19 June 2006; DOI: 10.1002/pi.2026

2006 Society of Chemical Industry. Polym Int 0959–8103/2006/$30.00

S Mondal, JL Hu

segment, Y = PTMG 2000, PBA 2000, or PCL 2000.For these PUs, water vapor permeability was shown toincrease in the following order of soft segment types:X = PBA < PCL < PTMG. An ester group is capableof much stronger interaction with water than an ethergroup; the strong decrease in water vapor permeability(almost −100%) observed with the polyurethane withsoft segment of PBA and PCL may be related tothe combined effect of an increased glass transitiontemperature and an accumulation of methylene groupsin the resulting PU. A comprehensive review of thepermeability through PUs was reported by Jonquiereset al.9 they stated that permeability in mixed softsegment block polyurethanes are rarely studied byresearchers.

In this study, three kinds of polyurethane blockcopolymer using mixed blocks, namely PCL 3000-PPG 3000, PCL 3000-PTMG 2900 and PPG 3000-PTMG 2900, were synthesized using a three-stepsynthesis process. In order to compare the influenceof the mixed block in polyurethane’s properties, anSPU with a single soft segment such as PCL 3000was also synthesized using a two-step polymerizationprocess. All SPUs were modified with PEG 3400 inorder to obtained enhanced water vapor permeability.Structures of all polyurethanes were studied byFTIR, wide-angle X-ray diffraction (WAXD), DSC,and dynamic mechanical thermal analysis (DMTA)techniques. Mass transfer properties were measuredby equilibrium sorption and water vapor permeabilitymeasurements.

EXPERIMENTALMaterialsPPG 3000 was obtained from International Labo-ratory, USA. All other chemicals were obtained fromAldrich. PPG 3000, PCL 3000, and PTMG 2900 werevacuum oven dried at 80 ◦C for 12 h, and PEG 3400was vacuum oven dried at 80 ◦C for 4 h prior to use.N,N ′-dimethylformamide (DMF) and 1,4-BDO weredried by 4 A molecular sieve. 4,4′-Diphenylmethanediisocyanate (MDI) was used as received.

SynthesisA 500 mL round-bottomed, three-necked flaskequipped with a mechanical stirrer, nitrogen inlet andthermometer was used as reactor. Polymers were syn-thesized using a two- or three-step synthesis processby solution polymerization technique in DMF solvent.

In order to obtain linear polymer, the molar ratio ofNCO to OH was kept at 1.0:1.0. The weight percentof hydrophilic segment, i.e. PEG 3400, was kept con-stant (10 wt%) for all cases. Polyurethane with PEG3400 and PCL 3000 (S21) was synthesized using atwo-step polymerization process. The other three, i.e.S18, S19, and S20, were synthesized using a three-steppolymerization process. In the two-step polymeriza-tion process, PEG 3400 and PCL 3000 (in samplecode S21) were reacted with MDI at about 80 ◦C for2 h to make the prepolymer and in the second stepchain extension was performed with 1,4-BDO at thesame temperature for another 2 h. In the three-steppolymerization process PEG 3400 and PCL 3000 (inS18 and S19), or PPG 3000 (in S20) was reacted with50 wt% of total MDI added at 80 ◦C for 1 h to makethe prepolymer I. In the second step, prepolymer Iwas reacted with PPG 3000 (S18) or PTMG 2900(S19 and S20), and the remaining 50 wt% of MDI atthe same temperature for 1 h to make prepolymer II.In the third step, chain extension was performed with1,4-BDO at the same temperature for another 2 h.The final polymer concentration in all cases was about20% (w/w). The compositions of all SPUs are given inTable 1.

Film preparationDense films were cast from diluted SPU (concentra-tion 5 w/v%) solution on Teflon-coated steel plate. Inorder to make defect-free non-porous films, solventwas evaporated at 60 ◦C for 12 h and residual solventwas removed at 80 ◦C for another 12 h in a vacuumoven. The film thickness for mass transfer propertieswas about 75–90 µm. About 0.2 mm thicker films wereused for mechanical testing and 15–20 µm film wasused for FTIR testing.

CharacterizationViscosity of the segmented polyurethane solutionin DMF was measured by modified Ubbelohdeviscometer (capillary diameter 0.6 mm) at 25 ◦C.

Perkin Elmer’s Fourier transform infrared (2000FTIR) was used to obtained the IR spectra of SPUsamples in the range 4000–400 cm−1.

The X-ray data were recorded by using Philipsanalytical X-ray (Philips Xpert XRD System) at avoltage of 40 V, 40 mA current and a radiation of1.542 A wavelength. Spectra were obtained in a rangeof Bragg’s angle 2θ = 10.10 − 40◦. The scanningspeed was 0.03 s per step. The degree of crystallinity

Table 1. Composition of SPUs

Feed (×10−3 mol)

Sample Polyol-I Polyol-II PEG 3400 MDI 1,4-BDO Wt% of PEG Intrinsic Viscosity (dL g−1)

S18 5 (PCL 3000) 5 (PPG 3000) 1.2 18.20 7 10 0.857S19 5 (PCL 3000) 5.17 (PTMG 2900) 1.2 18.37 7 10 1.125S20 5 (PPG 3000) 5.17 (PTMG 2900) 1.2 18.37 7 10 1.163S21 10 (PCL 3000) 0 1.2 18.20 7 10 0.916

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was calculated using the method described by Youngand Lovell.10 The area of the X-ray diffraction curvedue to scattering from the crystalline phase (Ac) andfrom the amorphous phase (Aa) can be separated, andthe percent crystallinity (Xc) within the sample can becalculated from the equation

Xc = Ac ∗ 100/(Ac + Aa) (1)

Differential scanning calorimetry data were obtainedusing a Perkin Elmer DSC 7. Each sample was scannedfrom −50 to 120 ◦C at a scanning rate of 10 ◦C min−1

under dry nitrogen purge. All runs were carried outwith a sample weight of 5–10 mg.

Dynamic mechanical thermal analysis (DMTA) wasperformed in tension mode using a Perkin ElmerDiamond DMA Lab System in the temperature range−100 to 120 ◦C under nitrogen purging at a frequencyof 2 Hz and a heating rate of 2 ◦C min−1.

The surface of the film was studied with scanningelectron micrographs (SEM) made with a LeicaStereoscan 440 equipped with an Oxford energydispersive X-ray system, operating at 20 kV.

Mass transfer properties were measured by equi-librium water sorption and water vapor permeability(WVP) measurements. The equilibrium sorption wasmeasured at five different temperatures, e.g. 10, 15,25, 35 and 45 ◦C. The WVP was measured accordingto the ASTM E 96-80B, at six different temperatures,namely 12, 18, 25, 35, 45 and 60 ◦C, and at constant70% relative humidity for 24 h. An average of threedifferent test results for each sample were reported,which was expressed in units of g m−2 24 h−1 orper day. The details procedure of sorption and WVPmeasurements are described elsewhere.1

RESULTS AND DISCUSSIONFourier-transform infrared (FTIR)The infrared spectrum of NH group stretching isshown in Fig. 1. The free NH (NHH-Free) group gavean IR absorption band at 3510–3530 cm−1, whereasthe signal for hydrogen-bonded (NHH-bonded)groups were seen at 3300–3347 cm−1. The peakposition and shape changed with composition of thesoft segment, which could originate from the factthat interaction among the hard and soft segmentsand soft–soft segments have been altered, throughhydrogen bonding and dipole–dipole interactionof carbamoyl group plus induced dipole–dipoleinteraction of the phenyl ring.11 An ester group iscapable of much stronger interaction than an ethergroup, which may be the reason for lower absorptionspectra of NHH-Free for S21. The shape of theNHH-bonded is sharp for single-block SPU (S21)and broadened when PPG 3000 (S18) or PTMG2900 (S19) was used along with PCL 3000 phase.The shape changes suggested weaken the hydrogenbonds. The phase separation in polyurethane can becharacterized by measuring the intensity and position

of N–H stretching vibration.12 The degree of phaseseparation is determined by the relative amounts ofhydrogen bonds between the hard and soft segment.13

Increased phase segregation favors inter-urethanehydrogen bonding. Therefore, S21 is more phaseseparated than other samples. Introduction of PPG3000 segment in either PCL 3000 or PTMG 2900phase increases phase mixing due to the asymmetricstructure of PPG 3000 block. The broad NHH-Freefor S20 indicates soft–soft segment phase mixingwhich would loosen the inter-chain interactions. TheCO residing in the small hard segment microdomainsolubilized in the polyether matrix phase are likelynon-hydrogen bonded14 as a result of the broadN–HFree band. It is interesting to note that a smallpeak near 3450 cm−1 has formed between NHH-Freeand NHH-bonded with the sample containing PCL3000; the new peak is obvious for sample S21, whichsuggested that the presence of the ester group wouldincrease the interaction between the polymer chains.

Wide-angle X-ray diffractionWAXD results at room temperature for pure polyoland resulting SPUs are shown in Figs 2 and 3.Polyurethanes for this study are composed of about80 wt% long polyol (soft segment). Because of

37003200 3300 3400 3500 3600

NHH-Free

New Peak

NHH-bonded

S19

S18

S21

S20

% T

Wave number (cm-1)

Figure 1. NH stretching of SPU samples.

10 15 20 25 30 35 40

S19

S18

S21

S20

L in

Cou

nts

2-Theta in degree

Figure 2. WAXD pattern of SPU samples.

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10 20 30 40

PTMG-2900

PCL-3000

L in

Cou

nts

2-Theta in degree

Figure 3. WAXD pattern of pure PCL 3000 and PTMG 2900.

their long polymer chains and ordered structures, asoft segment could form a crystalline structure inthe segmented block polyurethane.1 Therefore, fora better understanding, the WAXD results for purePTMG 2900 and PCL 3000 are shown in Fig. 3.However, PPG 3000 is liquid at room temperatureand WAXD could not be carried out. From Fig. 3, wecan see that PCL 3000 has two sharp diffraction peaksand the resulting SPUs (Fig. 2) based on PCL 3000also show two diffraction peaks, which signifies thepresence of ordered structure. The percent crystallinityof the resulting SPUs is lower than that of crystallinitycalculated from the weight fraction of pure polyoldue to the presence of hard segment, which acts asreinforcing filler and hinders the crystallization processof polyol.

The sample with only PCL 3000 (S21) has thehighest percent crystallinity among the four samplesowing to their phase-separated structure. With theintroduction of another soft segment block (PPG3000 or PTMG 2900) hindrance to the crystallizationof PCL 3000 phase due to soft–soft segment phasemixing makes the situation difficult for ordering ofthe PCL domain. The percent crystallinity of PCL3000-based SPUs, which was detected by WAXD andcalculated by Young’s equation,10 were in the orderof S21 (28.8%) > S19 (23.2%) > S18 (20%). FromFig. 3 we can see that SPU based on PTMG 2900and PPG 3000 has broad halos at 2θ of about 20◦,which may be due to the amorphous structure orpresence of small crystalline structure or diffractionfrom a large crystal.15 The DSC result confirms theendothermic peak; therefore the broad halo is due tothe presence of small crystallites which are scatteredthroughout the polymer matrix and could not bedetectable by WAXD. The percent crystallinity ofS20, which is 7.3%, could be roughly estimated fromthe interpretation of WAXD data and DSC results,and can be calculated from the following formula:1

(χPU%)/�HPU = (χPolyol%)/�HPolyol (2)

where χPU % and χPolyol % are the percent crystallinityof SPU and polyol, respectively; �HPU and �HPolyol

are the heats of fusion of SPU and polyol, respectively.

Table 2. DSC data of SPU

Sample�H

(J g−1)Tms

(◦C)�Hc

(J g−1)Tc

(◦C)

S18 29.4 51.2 3.2 −9.6S19 9.3, 30.4 19.2, 46.8 1.7, 28.2 −7.7, −17.7S20 −3.8, 4.4,

10.3−14.5, 10.9,

33.7– –

S21 52.5 55.5 33.8 −3.5

�H, heat of fusion; Tms, soft segment crystal melting temperature;�Hc, heat of crystallization; Tc, crystallization temperature; Tg, glasstransition temperature.

-25 0 25 50 75 100

Heating

CoolingE

ND

O

Temperature (°C)

Figure 4. DSC thermogram of sample S20.

-40 -20 0 20 40 60 80 100 120

S21

S19

S18

EN

DO

Temperature (°C)

Figure 5. DSC thermogram of SPU samples (S18, S19, S21).

Differential scanning calorimetrySoft segment crystal meting temperature (Tms) andheat of fusion (�H) of SPUs were obtained fromDSC measurements and are summarized in Table 2and shown in Figs 4–6. Each transition has beenattributed to a specific phenomenon. In DSC theamount of energy needed to maintain a fixed rate oftemperature is measured. Changes in heat of fusiondenote changes in the mobility of the polymer chains.13

All the resulting SPUs have a lower heat of fusionthan that calculated from the weight fraction of puresoft segment, due to phase mixing by hard and softsegments and soft–soft segment, which hinders thecrystallization process of crystallizable soft matrices.

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100806040200-20-40

S21S19

S18

EN

DO

Temperature (°C)

Figure 6. DSC cooling thermogram of SPUs (S18, S19, S21).

PCL 3000-based SPU samples have heats of fusionin the following order: S21 > S19 > S18. The reductionin crystallinity is due to the miscibility of PCL 3000and PPG 3000 segments which seems to be the causefor the decrease in the heat of fusion for sample S18.A sharp endothermic peak in sample S19 for PTMGphase appeared at 19.2 ◦C. The shapes of endothermicpeaks (S18 and S19) as well as exothermic peaks (S19)of mixed-block SPUs are different from that of singlesoft block SPU (S21) due, to some extent, to phasemixing altering the shape and position of transition.The heat of fusion of sample S20 (PPG 3000–PTMG2900) is the lowest among all four sample owing to thephase mixing of PTMG phase by PPG 3000 and finallydecreased order structure of PTMG 2900 phase. PPG3000 itself could not form a crystalline structure, whichis the reason for lower percent crystallinity and hencedecreased heat of fusion.

From the DSC cooling results, SPUs containingonly PCL 3000 and PCL 3000–PTMG 2900mixed soft block shows a sharp exothermic peak,signifying crystallization during the cooling cycle. Theexothermic peak was divided in two parts in the caseof sample S19, which contains the same quantity(weight content) of PCL 3000 and PTMG 2900blocks; this may be due to both phases separatelyforming a crystalline structure in the resulting phase-separated SPU. The presence of PPG 3000 preventedcrystallization of PCL 3000 (S18) phase as well asPTMG 2900 (S20) phase in the DSC cooling cycle,due to phase mixing by the PPG 3000 segment.

Dynamic mechanical thermal analysisDMTA is an important technique capable of providingconsiderable information on the position of transitionsand the thermomechanical properties of polymers.13

Figs 7 and 8 show the tensile storage modulus (E′)and dissipation factor (tan δ) of SPUs, respectively, asa function of temperature. The glassy-state storagemodulus of S20 is higher than that of the othersamples owing to the presence of a flexible softsegment (PTMG 2900 and PPG 3000) in the polymerbackbone and nearly amorphous structure which

-100 -50 0 50-5.00E+0080.00E+0005.00E+0081.00E+0091.50E+0092.00E+0092.50E+0093.00E+0093.50E+0094.00E+0094.50E+0095.00E+009

S18S19S20S21

E ′(

Pa)

Temperature (°C)

Figure 7. Tensile storage modulus of SPU samples.

-100 -50 0 500.00

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

S18S19S20S21

Tan

(d)

Temperature (°C)

Figure 8. Tan δ results of SPU samples.

would increases the glassy-state modulus. There isa dramatic fall in storage modulus of S20 at Tg, whichis quite obvious as compared to the other samplesbecause of the presence of a flexible backbone, whichwould increase chain mobility above Tg. The order ofstorage modulus of PCL 3000-based SPUs is E′

S21 >

E′S18 > E′

S19. An increased amount of crystallinityby reduced phase mixing and stronger inter-chaininteraction of the ester group is the reason for higherglassy-state modulus in sample S21. Introduction offlexible PPG 3000 or PTMG 2900 segment in PCL3000 based SPU was decreases the storage modulus.Storage modulus of S18 is slightly higher than the S19,linear and flexible PTMG 2900 segment increases thechain mobility and hence lower glassy-state modulus.However in case of S18, the chain mobility becomes abit tougher due to the presence of methylene group inPPG 3000.

The temperature associated with the peak mag-nitude of tan δ is defined as the glass transitiontemperature (Tg), and its height and shape provideinformation about the degree of order and freedomof molecular mobility of the soft segment. In addi-tion, crystallization behavior of the polymers can bedescribed by the shape of the tan δ peak.16 Sincetan δ peaks are not sharp, it is difficult to measurethe exact Tg of corresponding samples; however, wecould get a rough idea of Tg from the tan δ peak(Fig. 8). The glass transition temperature of S20 (PPG

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3000–PTMG 2900) is the lowest among the foursamples, owing to their flexible and phase-mixed struc-ture, which increases the chain mobility that broughtdown the Tg. The tan δ values for this sample arehigher than for the other samples, indicating soft handfeeling or better tactile properties suitable for tex-tile applications. The loss tan δ is unclear for S21

owing to the strong interaction by ester groups, whichmakes segmental mobility difficult at glass transi-tion temperature. Higher inter-chain interactions andcrystallization (WAXD and DSC results) lowered itsflexibility as a result of low tan δ11 for this sample (S21).Thermodynamic incompatibility between the soft andhard segments becomes more distinct, implying thatthe material becomes more phase separated.17 Thetan δ value increased and glass transition temperaturedecreased when PPG 3000 (S18) or PTMG 2900 (S19)was used along with PCL 3000 soft segment as mixedblock, owing to the increasing chain flexibility andto some extent phase mixing, which makes facilitatessegmental mobility. The tan δ value of S19 is lowerthan that of S18. In S19, both PCL 3000 and PTMGphases are crystallizable in the pure state and form acrystalline structure in the resulting SPU (S19); theincrease of soft segment crystallites would probablyrestrict the chain mobility as a result of lower tan δ.

Mass transfer propertiesNon-porous membranes are dense, pinhole-free poly-mer membranes (Fig. 9), and the mass transfer ofthis kind of membrane occurs in molecular mecha-nisms (sorption–diffusion–disorption) that depend onthe primary structure of the polymer itself and poly-mer–penetrant interaction.6 We characterize the masstransfer properties of non-porous SPU membranesby equilibrium sorption and water vapor permeabil-ity measurements. The results are discussed in thefollowing sections.

SorptionEquilibrium sorption of all samples were influencedby the microstructure of SPUs. Table 3 shows thatthe water sorption of S20 is the highest among the

Figure 9. Non-porous structure of SPU (S20).

four samples; the equilibrium sorption of this samplewas influenced by the presence of a hydrophilicsegment, i.e. PEG 3400, and an almost amorphousand loose structure which would provide more spacefor water vapor molecules. As the hydrophilicity ofthe membrane increases, the void volume betweenthe molecules also increases and drug diffusionbecomes easier. Water sorption for all samples wasenhanced with increasing temperature. Once watermolecules were absorbed by the polymer chains, thiscould increase the chain mobility with increasingtemperature due to the plasticization effect of watermolecules and polymer chains, and provide morespace for water molecules in the polymer membranes.In addition, increasing temperature increases the freevolume, which also enhances the equilibrium sorption.The sorption of sample S21 is the lowest among thefour samples owing to the presence of the ester group,which will possess strong inter-chain interaction andhinders the equilibrium water sorption. The sorptioncoefficient decreases with the introduction of polyestersegment, which signifies less affinity of water moleculeswith the polyester moiety. The whole phenomenoncan be explained on the basis of crosslink density andnetwork composition. As PCL 3000 was introducedinto the SPU backbone, which possesses stronginter-chain interaction between the polymer chains(FTIR results), these interchain interactions actedas a physical crosslinking. As the crosslink densityincreases and the chains become more rigid and dense,the ability of polymer chains to accommodate watermolecules decreases. Sorption of PCL 3000-basedSPU increased when PPG 3000 or PTMG 2900 wasused along with PCL 3000, due to the increasedchain mobility. When the experimental temperatureis reached to soft segment crystal melting pointwhich causes discontinuous density changes inside themembrane and sorption would be further enhanced.

Water vapor permeabilityWVP results show the same trends as equilibriumwater sorption behavior of the corresponding SPUsamples. From the DMTA results we observed that theglass transition temperature of all SPU samples werewell below the experimental temperature of WVP test-ing; therefore the fractional free volume increased withincreasing temperature18 at experimental temperatureaccording to Eqn (3), which provided more paths forwater molecules to pass through the membrane:

Fractional free volume (FFV)

= fg + (α1 − α2) × (T − Tg) (3)

Table 3. Equilibrium sorption (%) data of SPUs

Sample 10 ◦C 15 ◦C 25 ◦C 35 ◦C 45 ◦C

S18 13.18 14.57 16.25 17.15 18.39S19 12.44 13.1 13.55 14.15 15.87S20 20.17 21.50 22.85 24.69 25.85S21 6.72 7.18 8.30 8.88 9.35

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Table 4. Water vapor permeability data of SPUs (g m−2 24 h)

Sample 12 ◦C 18 ◦C 25 ◦C 35 ◦C 45 ◦C 60 ◦C

S18 215 290 420 511 850 1308S19 124 310 488 605 955 1145S20 367 465 615 900 1450 1828S21 115 155 272 337 405 1464

10 20 30 40 50 600

200

400

600

800

1000

1200

1400

1600

1800

2000

Wat

er v

apo

r p

erm

eab

ility

(g

m-2

24

h-1

)

Temperature (°C)

S18S19S20S21

Figure 10. WVP of SPU samples.

where α1 and α2 are the thermal expansion coefficientsin the rubbery and glassy states, respectively, fg is thefractional free volume at Tg, which is constant (0.025),and T is the experimental temperature. In addition,the increase of WVP with increasing temperature isdue to the higher energy in the membrane segment athigher temperature, which will break or weaken someof intermolecular interactions among the individualsegments within the polymer; this results in thematrix being more fluid in nature and enhances thepermeability.19

WVP of S20 (PPG 3000–PTMG 2900) is thebest among the four samples (Table 4 and Fig. 10)regardless of temperature. The presence of flexiblePPG and PTMG segments in the polymer backboneand their phase mixing in the resulting SPUs, whichincreases chain mobility, are the reasons for higherWVP in this sample. The low crystallinity and flexiblestructure give a loose amorphous20 structure throughwhich water vapor molecules can easily pass. TheWVP was further enhanced when the experimentaltemperature reached the soft segment crystal meltingpoint (Tms). At and above Tms WVP was a confluenceof chain flexibility, hydrophilicity, and soft segmentcrystal melting temperature, which would causediscontinuous density changes inside the membranes.Owing to their combined effect, WVP increasedabruptly for S20. The WVP rate of S21 (PCL 3000)was lowest up to 45 ◦C among the four samples, dueto the presence of ester groups, which increased theinter-chain interaction and prevented water moleculesfrom passing through the membrane. The percentcrystallinity of this sample was highest among the foursamples; therefore, the combination of stiff characterof polymer chains and high percent crystallinity in the

soft segments may create a polymeric ‘molecular sieve’,as with the semi-crystalline polymer,21 as a result oflower permeability before soft segment crystal melting.Therefore, before soft segment crystal melting theincrease of WVP with increasing temperature is dueto the increasing free volume according to Eqn (3).The abrupt change in WVP observed, from 45 to60 ◦C, was due to crystal melting. Since the percentcrystallinity of S21 was highest among the four samples,their melting abruptly enhanced the WVP, which isthe reason for the higher water vapor permeabilityof S21 at 60 ◦C compared with other two samples(S18 and S19). When PPG 3000 (S18) or PTMG2900 (S19) was used as a mixed block along withPCL 3000, the permeability of the resulting SPU atlow temperature (12–25 ◦C) increased significantlydue to the enhanced chain flexibility as comparedto SPU containing only PCL 3000 segment. Oncethe experimental temperature reached about 19 ◦C,the crystal melting of PTMG 2900 phase furtherenhanced the permeability; however, the Tms of theother phase, i.e. PCL 3000, did not influence the WVPat that temperature. The micro-Brownian motion ofthe soft segment at crystal melting point temperature(Tms) obviously increased the intermolecular gap largeenough to allow water vapor molecules to pass throughthe membrane.18 A high WVP was observed for S18

from 45 to 60 ◦C due to the soft segment crystalmelting, which is about 51 ◦C. Therefore, the increaseof WVP for S18 from 45 to 60 ◦C is higher than for S19.

CONCLUSIONSFrom FTIR results, SPU block copolymer with PCLsoft segment shows strong inter-chain interaction,which was loosened by using PPG or PTMG alongwith PCL soft segment. If the pure soft segment iscrystallizable, then it could form a crystalline structurein the resulting SPU separately when used as amixed soft segment block, detected by WAXD andDSC. Dynamic mechanical thermal properties werealso influenced by selection of the soft block. PCL-based SPU enables the polymer matrix to sustaina high modulus value at higher temperature. Thehighest loss tangent was obtained with a mixedblock of PTMG and PPG-based SPU owing totheir loose, flexible and nearly amorphous structure,suggesting the importance of the combination of mixedsoft segment block. The behavior of mass transferproperties in SPU membranes can be interpretedon the basis of the chemical nature of the softsegment in the SPU backbone, changes in crystallinity,hydrophilicity and/or Tg in these kinds of phase-separated structure. SPU with mixed block containingPTMG 2900 and PPG 3000 block performed bestwith water sorption as well as water vapor permeability.Soft segment crystal melting in the experimentaltemperature range enhances permeability due toincreased chain mobility and discontinuous densitychanges inside the membrane.

Polym Int 55:1013–1020 (2006) 1019DOI: 10.1002/pi

S Mondal, JL Hu

ACKNOWLEDGEMENTSThe authors gratefully acknowledged the InternationalPostgraduate Scholarship of the Hong Kong Polytech-nic University, Hong Kong, for providing financialassistance for this investigation.

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